U.S. Pat. No. 4,256,796 discusses the developmemt of electronic circuit boards consisting basically of a substrate coated with a dielectric material upon the surface of which the required conductive metal circuitry is formed. The substrates or circuit boards, as such have been commonly termed, have been fabricated from organic plastics, ceramic wafers, and porcelain-coated steel. Each of those materials displays certain deficiencies.
For example, organic plastics suffer from low temperature capability, i.e., they cannot be subjected to temperatures greater than about 400.degree. C. That circumstance limits the manufacturing process for applying the metal circuitry upon the surface thereof to low temperature techniques. Newer methods for applying the circuitry have been developed which are lower in cost and provide improved reliability and electrical accuracy, but which demand firing temperatures in excess of 600.degree. C. Such new processes are not applicable with substrates prepared from organic plastics.
Ceramic substrates, such as alumina wafers, can be repeatedly exposed to temperatures in excess of 600.degree. C. without failure but the materials are expensive, difficult to fabricate into large and complex shapes, and cannot be readily machined or have holes punched therein. Moreover, the wafers have proven to be relatively fragile.
Porcelain-coated steel boards have experienced the problem of unevenness in the thickness of the porcelain layer. Furthermore, the available porcelains were subject to flow at temperatures of 500.degree.-600.degree. C., brown plague, blisters, pinholes, and poor adhesion to the steel substrate because of substantial differences existing between the coefficients of thermal expansion of the porcelain and the steel board.
U.S. Pat. No. 4,256,796 discloses the fabrication of porcelain-coated, metal circuit boards wherein the "porcelain" is a devitrified glass having a composition which is substantially free from alkali metal oxide and consists essentially, expressed in terms of mole percent on the oxide basis, of
______________________________________ BaO 6-25 MgO + optionally CaO and/or ZnO 30-60 B.sub.2 O.sub.3 13-35 SiO.sub.2 10-25 ______________________________________
Although a direct conversion of ranges expressed in mole percent to ranges stated in weight percent is not possible, the following table provides an approximation of the disclosed compositions (also utilizing the working examples of the patent) in weight percent.
______________________________________ BaO 16-50 MgO 16-42 CaO 0-11 ZnO 0-11 CaO + ZnO 0-11 B.sub.2 O.sub.3 12-34 SiO.sub.2 10-23 ZrO.sub.2 0-5 Al.sub.2 O.sub.3 0-5 SnO.sub.2 0-5 ZrO.sub.2 + Al.sub.2 O.sub.3 + SnO.sub.2 0-5 ______________________________________
The patent describes melting batches for the above compositions, forming glass frits from the melts, applying a coating of the frits onto the surface of a metal substrate (most preferably low carbon steel), and then firing the coated substrate at a temperature of at least 750.degree. C., more desirably 800.degree.-850.degree. C., for a sufficient length of time to cause the frit to essentially simultaneously fuse together to an integral mass and crystallize in situ, this time commonly ranging about 5-30 minutes. The resultant coating is highly crystalline, 50-90% by volume, with the remainder consisting of residual glass. The primary crystal phase comprises BaO.2MO.2SiO.sub.2, wherein MO consists of MgO with, optionally, CaO and/or ZnO, and the secondary crystal phase comprises 2MO.B.sub.2 O.sub.3, wherein MO again consists of MgO with, optionally, CaO and/or ZnO. The crystallized coatings demonstrate high coefficients of thermal expansion, typically 110.times.10.sup.-7 /.degree. C. and above, over the range of 0.degree. C. to the deformation temperature of the material (&gt;700.degree. C.). Such coefficients of thermal expansion are useful in matching the high coefficients of thermal expansion of the metal substrates employed, e.g., the preferred low carbon steels.
Although those patented compositions provide coatings far superior to the previously-available porcelain coatings, the need has been observed for materials exhibiting even greater refractoriness, better electrical properties, and, desirably, lower coefficients of thermal expansion for use with substrates prepared from alloys manifesting expansion coefficients lower than those of low carbon steels. Therefore, the primary objective of the instant invention is to provide crystallized glass or glass-ceramic coatings which satisfy that need.